1. Field of the Invention
The present invention relates to a control device and a control method for an internal combustion engine, and more particularly relates to a control device and a control method for an internal combustion engine, which cause after-burning of unburned HC in the exhaust gases within the exhaust passage by supplying secondary air upstream of a catalyst in the exhaust passage.
2. Description of the Related Art
As described in, for example, Japanese Patent Application Publication No. JP-A-2001-263050, there is a per se known technique of, when starting an internal combustion engine, supplying external air taken in by an air pump as secondary air into the exhaust passage upstream of a catalyst, and thereby causing unburned HC (rich components) in the exhaust gas to be after-burned within the exhaust passage. With the technique described in this Japanese Patent Application Publication No. JP-A-2001-263050, the combustion in the internal combustion engine is positively controlled according to the ignition timing or the like, so that the exhaust temperature reaches at a level at which after-burning can take place: and secondary air is supplied into the exhaust passage.
The amount of secondary air which is supplied is determined by the rotational speed of the air pump and the pressure within the exhaust passage, and, if the rotational speed of the air pump is constant, the amount of secondary air which is supplied becomes less, the higher is the pressure within the exhaust passage. However, with the above described technique, no consideration is given to the pressure within the exhaust passage when supplying the secondary air. Due to this, with the above described technique, sometimes a situation arises in which, because the pressure within the exhaust passage is high, it is not possible to supply a sufficient amount of secondary air. In this type of situation, it is not possible to cause sufficient after-burning of the unburned HC within the exhaust passage, and there is a possibility that this unburned HC will be discharged into the atmosphere just as it is, which is undesirable. Furthermore, since sufficient after-burning to raise the exhaust temperature is not done, sometimes it takes an unduly long time for the catalyst to become warmed up. In other words, with the above described technique, the possibility of reliably obtaining the beneficial effects of after-burning by supply of secondary air has not been fully realized, and there is as yet room for improvement in enhancement of the exhaust emissions when starting the internal combustion engine, in particular during starting of the engine from cold.